Mihail HRS :
I want to call a print function from my C program.
assembler prog:
#test.s
.text
.global _start
.global print
.type print, @function
_start:
call print
# and exit.
movl $0,%ebx # first argument: exit code.
movl $1,%eax # system call number (sys_exit).
int $0x80 # call kernel.
print:
# write our string to stdout.
movl $len,%edx # third argument: message length.
movl $msg,%ecx # second argument: pointer to message to write.
movl $1,%ebx # first argument: file handle (stdout).
movl $4,%eax # system call number (sys_write).
int $0x80 # call kernel.
mov $0, %eax
ret
.data
msg:
.ascii "Hello, world!\n" # the string to print.
len = . - msg # length of the string.
I can assemble and link it using:
$as test.s -o test.o
$ld test.o -o test
And I can execute it as a program, and it outputs "Hello, world!" But when I tried to call a print from C code like this:
#include <stdio.h>
extern int print();
int main(){
int g;
g = print();
printf("Hello from c!, %d\n", g);
}
It was compiled using:
$gcc -c main.c test
It just prints "Hello from c, 13", that means that the function was called and return a number of chars, but does not print anything!
What am I doing wrong?
P.S. When I trying to compile prog like this:
$as test.s -o test.o
$gcc -c main.c -o main.o
$gcc main.c test.o
I have a error:
/usr/bin/ld: test.o: in function `_start':
(.text+0x0): multiple definition of `_start'; /usr/lib/gcc/x86_64-pc-linux-gnu/9.2.0/../../../../lib/Scrt1.o:(.text+0x0): first defined here
/usr/bin/ld: test.o: relocation R_X86_64_32 against `.data' can not be used when making a PIE object; recompile with -fPIE
/usr/bin/ld: final link failed: nonrepresentable section on output
collect2: error: ld returned 1 exit status
Mihail HRS :
Ok, done! Thanks clearlight
I can compile all use
$as test.s -o test.o
$gcc -c main.c -o main.o
$gcc -no-pie main.c test.o
And all will work fine!